1. Field of the Invention
The present invention relates to a car control method and, specifically, to a car control method for suitably controlling drive or braking force applied to wheels and suppressing disturbance applied to tires and an apparatus for use in the method.
2. Description of the Prior Art
In the prior art, to control the running state of a car by controlling drive force applied to drive wheels, in the case of an engine-drive car, the target engine speed is calculated based on the input acceleration signal, and the opening of a throttle valve is controlled to adjust the engine speed so that the actual car speed detected by a car speed sensor becomes a car speed calculated from the above target engine speed so as to control drive torque applied to an output shaft connected to the drive wheels.
In an electric car in which drive wheels are driven or braked by an electric motor, as shown in the control flow of FIG. 12, the output torque of an electric motor 50M for driving or braking drive wheels 50R is detected by drive torque detection means 51, the wheel speed is detected by a wheel speed sensor 52, the target wheel speed in a sticky state is calculated from the target drive force (motor torque instruction value) for obtaining required drive torque and the detected output torque of the motor by target wheel speed calculating means 53 so that the above detected wheel speed becomes the above target wheel speed, and drive or braking force generated by the above electric motor 50M is controlled by motor drive and control means 54 to adjust drive or braking force applied to the drive wheels 50R. At this point, the ratio of the above wheel torque to car body drive force is controlled to become equal to the ratio of the mass of the wheels to the mass of the car body. The motor drive and control means 54 controls the size of a current running through the above electric motor 50M by detecting the torque of the output shaft of the motor or directly detecting a motor current.
When the vibration of the car is caused by fluctuations in the output torque of the electric motor 50M, as means of controlling this vibration, for example, there are proposed a method in which motor torque is detected as a motor revolution angle θm or a motor revolution speed ωm, the difference “e” between this θm or ωm and the estimated value θme of the motor revolution angle or the estimated value ωme of the motor revolution speed based on a plant model is calculated and disturbance torque is estimated from this difference “e” to control motor torque to be input into the actual plant (for example, see Japanese Laid-open Patent Application No. 2000-217209) and a method in which the difference Δω between the average revolution ωM of the motor and the average revolution ωb of the drive wheels is calculated by detecting these and a torque instruction value τM to be input into the motor is corrected with a correction value τ′ obtained by multiplying this difference Δω by a gain k (for example, see Japanese Laid-open Patent Application No. 2002-152916).
For a car having an SR motor driven by a switching circuit, there is also proposed a method in which a signal having a resonance frequency band is extracted from a motor revolution signal detected from the motor by a bandpass filter and fed back to reduce fluctuations in the revolution of a motor shaft caused by the vibration of the motor due to the resonance of the motor or the car body (for example, see Japanese Laid-open Patent Application No. 2002-171778).
In the above car control, to control the attitude of the car or car speed detected by a yaw rate sensor or car speed sensor, drive or braking force applied to the drive wheels is controlled according to the time constant of the behavior of the car to be controlled.
Stated more specifically, a high-frequency component which becomes a noise component to the behavior of the car is removed by a low-pass filter in the loop to control drive or braking force applied to the drive wheels at a control cycle of about 100 msec to 10 sec in an engine-drive car or at a control cycle of about 1 msec to 10 sec in an electric car.
When the car runs on a road having a low coefficient of friction, the running state of the car is controlled based on the estimated coefficient of road friction. As means of estimating the coefficient of road friction, for example, there is proposed a method in which the vibration of the wheel speed is extracted to improve the accuracy of response output to an input into a wheel resonance system including friction characteristics between the tires and the road surface, only a cyclic vibration is selected from this vibration of the wheel speed, and the transmission function of the wheel resonance system is identified based on this cyclic vibration (see Japanese Laid-open Patent Application No. 11-178120).
To suppress the spinning of the wheels when a car suddenly starts to run on a road having a low coefficient of friction, as shown in FIG. 12, the car body speed is detected by a car body speed sensor 55, the slip ratio which is determined by the speed difference between the wheel speed and the car body speed generated by the spinning of the wheels from the above detected car body speed and the wheel speed detected by the wheel sensor 52 is calculated by slip ratio calculating means 56, and the above target wheel speed is corrected so that the above slip ratio becomes a preset slip ratio to suppress a rise in the above slip ratio, thereby shorting the drive or braking distance. Since drive or braking force applied to the drive wheels is controlled based on information on the behavior of the car, this control is also carried out at the same control frequency.
There is also proposed a method in which the acceleration of the drive wheels is detected by an acceleration sensor in place of the slip ratio calculated from the speed difference between the car body speed and the wheel speed and it is judged whether the car slips or not from the relationship between the output of this acceleration sensor and an increase in the revolution of the drive wheels. When it is judged that the car slips, the motor torque is reduced (for example, see Japanese Laid-open Patent Application No. 11-178120).
There is further proposed a method in which, by applying micro-vibration having a higher frequency than the response frequency of the car body to the tires according to the condition of the road in order to change friction force between the tires and the road surface, the running state of the car is controlled by adjusting the friction force of the tires while the slip ratio or slip angle of the tires is kept constant (for example, see the pamphlet of WO02/00463).
However, since the influence of disturbance (micro-vibration) applied to the tires caused by fluctuations in the contact pressure of the tires due to changes in the condition of the road and changes in the vibration of unsprung mass including the suspension and having a cycle of 100 msec or less cannot be compensated for by the general control of the above engine-drive car, the road holding properties of the tires deteriorate. Meanwhile, in the electric car, although shorter cyclic control than that of the engine-drive car is possible, a higher frequency band fluctuating component than the response frequency of the car body included in the above detected motor torque and wheel speed cannot be specified, thereby making it impossible to compensate for the influence of disturbance applied to the tires.
Since only the friction force of the tires is controlled in the method in which micro-vibration is directly applied to the tires, it cannot be said that the control of the attitude of the car and the car speed is satisfactory.
Further, as the control of the attitude of the car and the car speed and the control of the slip ratio are carried out at the same control frequency range, over-feedback occurs for the control of the attitude of the car and the car speed. Thus, satisfactory car control is impossible.
It is an object of the present invention which has been made in view of the above problems of the prior art to provide a car control method capable of improving the road holding properties of the tires by compensating for disturbance applied to the tires and the driving stability of the car and an apparatus used in the method.